Scatter plots in MATLAB - Step-by-Step Execution

Concept Flow - Scatter plots

Prepare data vectors X and Y

↓

Call scatter(X, Y)

↓

Plot points on graph

↓

Customize plot (color, size, labels)

↓

Display scatter plot window

Scatter plots show points on a graph using X and Y data vectors, then customize and display the plot.

Execution Sample

MATLAB

X = [1, 2, 3, 4];
Y = [10, 15, 13, 17];
scatter(X, Y, 'filled');
title('Simple Scatter Plot');
xlabel('X axis');
ylabel('Y axis');

This code creates a scatter plot of four points with filled markers and labels the axes and title.

Execution Table

Step	Action	Data State	Plot State	Output
1	Create X vector	X = [1 2 3 4]	No plot yet	No output
2	Create Y vector	Y = [10 15 13 17]	No plot yet	No output
3	Call scatter(X, Y, 'filled')	X and Y unchanged	Points plotted at (1,10), (2,15), (3,13), (4,17) with filled markers	Scatter plot points visible
4	Add title	Data unchanged	Title set to 'Simple Scatter Plot'	Title displayed on plot
5	Add xlabel	Data unchanged	X axis labeled 'X axis'	X axis label visible
6	Add ylabel	Data unchanged	Y axis labeled 'Y axis'	Y axis label visible
7	Display plot window	Data unchanged	Complete scatter plot shown	Graph window visible with points and labels

💡 All commands executed, scatter plot fully displayed

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
X	undefined	[1 2 3 4]	[1 2 3 4]	[1 2 3 4]
Y	undefined	undefined	[10 15 13 17]	[10 15 13 17]

Key Moments - 3 Insights

Why do the points appear only after calling scatter(X, Y)?

Because creating vectors X and Y only stores data; the scatter function actually draws the points on the plot (see execution_table step 3).

What does the 'filled' option do in scatter(X, Y, 'filled')?

It fills the markers with color instead of just showing outlines, making points easier to see (refer to execution_table step 3).

Why do we add title, xlabel, and ylabel after scatter?

These commands add labels and titles to the existing plot for clarity, enhancing the plot after points are drawn (see steps 4-6).

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the state of the plot?

A No points plotted yet

B Only axes labels are set

C Points plotted with filled markers

D Plot window closed

💡 Hint

Check the 'Plot State' column at step 3 in execution_table

At which step does the X axis label appear on the plot?

A Step 5

B Step 3

C Step 2

D Step 7

💡 Hint

Look for 'X axis labeled' in the 'Plot State' column in execution_table

If we remove the 'filled' option from scatter, what changes in the plot?

A Points become larger

B Points are hollow (not filled)

C Plot title disappears

D X and Y vectors change

💡 Hint

Recall the explanation in key_moments about the 'filled' option

Concept Snapshot

Scatter plots in MATLAB:
- Use scatter(X, Y) to plot points from vectors X and Y.
- 'filled' option fills markers with color.
- Use title(), xlabel(), ylabel() to add labels.
- Points appear only after scatter() is called.
- Customize plot after points are drawn.

Full Transcript

This visual execution shows how MATLAB creates a scatter plot step-by-step. First, vectors X and Y are created to hold data points. Then, scatter(X, Y, 'filled') draws the points on the graph with filled markers. After that, title, xlabel, and ylabel commands add labels to the plot for clarity. The plot window finally displays all these elements together. Key moments include understanding that data vectors alone do not show points until scatter is called, and the 'filled' option changes marker style. The quiz questions help check understanding of plot state at each step and the effect of options.

Practice

1. What does a scatter plot in MATLAB primarily show?

easy

A. A bar chart of grouped data

B. A continuous line connecting data points

C. Points representing pairs of values on an X-Y graph

D. A pie chart of category percentages

Solution

1. Step 1: Understand scatter plot purpose

   A scatter plot shows individual points for pairs of values, usually X and Y coordinates.

2. Step 2: Compare options to definition

   Only Points representing pairs of values on an X-Y graph describes points on an X-Y graph, which matches scatter plot behavior.

3. Final Answer:

   Points representing pairs of values on an X-Y graph -> Option C

4. Quick Check:

   Scatter plot = points on X-Y graph [OK]

Hint: Scatter plots show points, not lines or bars [OK]

Common Mistakes:

• Confusing scatter plot with line plot
• Thinking scatter plots show bars or pie slices
• Assuming scatter plots connect points

2. Which of the following is the correct MATLAB syntax to create a scatter plot of vectors x and y?

easy

A. plot.scatter(x, y)

B. scatter(x, y)

C. scatterplot(x, y)

D. scatter x, y

Solution

1. Step 1: Recall MATLAB scatter syntax

   The correct function to create scatter plots is scatter(x, y).

2. Step 2: Check other options for syntax errors

   Options A, C, and D are not valid MATLAB commands for scatter plots.

3. Final Answer:

   scatter(x, y) -> Option B

4. Quick Check:

   scatter function syntax = scatter(x, y) [OK]

Hint: Use scatter() function with parentheses and commas [OK]

Common Mistakes:

• Using dot notation like plot.scatter
• Typing scatterplot instead of scatter
• Omitting parentheses or commas

3. What will the following MATLAB code display?

x = 1:5;
y = [2 4 6 8 10];
scatter(x, y, 100, 'r', 'filled')

medium

A. A scatter plot with red filled circles sized 100 at points (1,2), (2,4), ..., (5,10)

B. A line plot connecting points from (1,2) to (5,10)

C. An error because 'filled' is not a valid option

D. A scatter plot with default size and blue color points

Solution

1. Step 1: Understand scatter parameters

   The code uses scatter(x, y, 100, 'r', 'filled') which sets point size to 100, color red ('r'), and fills the markers.

2. Step 2: Interpret the plot output

   The plot shows red filled circles at points (1,2), (2,4), (3,6), (4,8), and (5,10).

3. Final Answer:

   A scatter plot with red filled circles sized 100 at points (1,2), (2,4), ..., (5,10) -> Option A

4. Quick Check:

   scatter(x,y,100,'r','filled') = red filled circles [OK]

Hint: Size, color, and fill control marker appearance [OK]

Common Mistakes:

• Thinking 'filled' causes error
• Confusing scatter with line plot
• Ignoring size and color parameters

4. Identify the error in this MATLAB code for a scatter plot:

x = 1:4;
y = [5 6 7];
scatter(x, y)

medium

A. Vectors x and y have different lengths causing an error

B. scatter requires a third argument for size

C. scatter cannot plot numeric vectors

D. No error; code runs fine

Solution

1. Step 1: Check vector lengths

   Vector x has length 4, but y has length 3, which is invalid for scatter.

2. Step 2: Understand scatter input requirements

   Scatter requires x and y to be the same length to plot pairs of points.

3. Final Answer:

   Vectors x and y have different lengths causing an error -> Option A

4. Quick Check:

   Length mismatch in x,y vectors = error [OK]

Hint: x and y must be same length for scatter [OK]

Common Mistakes:

• Assuming scatter auto-fixes length mismatch
• Thinking size argument is mandatory
• Believing scatter can't plot numeric data

5. You have two groups of data points:

x1 = [1 2 3]; y1 = [4 5 6];
x2 = [1 2 3]; y2 = [7 8 9];

How can you plot both groups on the same scatter plot with different colors using filled markers and add a legend?

hard

A. plot(x1, y1, 'b'); plot(x2, y2, 'r'); legend('Group 1', 'Group 2')

B. scatter(x1, y1, 'b', 'filled'); scatter(x2, y2, 'r', 'filled'); legend('Group 1', 'Group 2')

C. scatter(x1, y1, 'b'); hold on; scatter(x2, y2, 'r'); legend('Group 1', 'Group 2')

D. scatter(x1, y1, 'b', 'filled'); hold on; scatter(x2, y2, 'r', 'filled'); legend('Group 1', 'Group 2')

Solution

1. Step 1: Plot first group with filled blue markers

   Use scatter(x1, y1, 'b', 'filled') to plot blue filled points.

2. Step 2: Use hold on before plotting second group

   To overlay plots, hold on keeps the first plot visible before adding the second.

3. Step 3: Plot second group with filled red markers and add legend

   Plot second group with scatter(x2, y2, 'r', 'filled') and add legend for clarity.

4. Final Answer:

   scatter(x1, y1, 'b', 'filled'); hold on; scatter(x2, y2, 'r', 'filled'); legend('Group 1', 'Group 2') -> Option D

5. Quick Check:

   Use hold on to overlay scatter plots with legend [OK]

Hint: Use hold on to combine scatter plots with different colors [OK]

Common Mistakes:

• Forgetting hold on causes second plot to overwrite first
• Using plot instead of scatter for points
• Omitting legend or using incorrect syntax